Custom Domains for AWS Lambda/API Gateway using Letsencrypt

AWS Lambda lets you run code without provisioning or managing servers. You pay only for the compute time you consume - there is no charge when your code is not running.

In general Lambda is well designed and the platform is pretty developer friendly, especially if you use a framework like serverless or apex. However as someone who creates new services on Lambda all the time, there is one thing that consistently annoys me.

Configuring a custom domain for use with Lambda is stupidly complex for such a common feature.

Here’s the AWS documentation to use a custom domain with API Gateway. Take a look, I’ll wait.

At first glance the instructions seem somewhat reasonable. For security reasons API Gateway requires SSL for all requests, which means that to use a custom domain, you first need an SSL certificate.

Unfortunately this becomes a problem when you realize that Letsencrypt HTTP-01 doesn’t work because of the catch-22 requiring you to prove that you own the custom domain before generating certificates. Even worse, AWS’s built-in free certificate service (Certificate Manger) doesn’t yet support API Gateway.

So what’s the solution?

I was able to create a nice little script using python which invokes the aws-cli, dehydrated letsencrypt client & lexicon and does all the steps necessary to add a custom domain to an API Gateway, automatically.

Here’s what it does:

  • validates that all the correct credentials & environmental variables are set
  • validates that the specified AWS API Gateway exists
  • generate a new set of letsencrypt certificates for the specified custom domain using the DNS-01 challenge & lexicon
  • register custom domain name with AWS (which creates a distribution domain name on cloudfront)
  • adds a CNAME dns record mapping your custom domain to the AWS distribution domain
  • maps the custom domain to your selected API Gateway

The code is all open source and lives here: Analogj/aws-api-gateway-letsencrypt

I’ve also created a simple Docker image which you can use if you don’t want to install anything:

docker run \
-e \
-e API_GATEWAY_NAME=dev-quietthyme-api \
-v `pwd`/certs:/srv/certs \

Jason Kulatunga

Build Automation & Infrastructure guy @Adobe. I write about, and play with, all sorts of new tech. All opinions are my own.

San Francisco, CA

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